I was my own contractor on a 3000 sq feet concrete house project. It is doable but you are not giving enough information. Cost will depend on hundreds of factors. First you would need to know where are you building and the regulations in your area. Will you need enginnering plans and permits or you have no building codes (very few places in the USA with no building codes).How much and how easy is to buy cement and associated materials and tools in your area? How much you expect to expend? What are your skills and how old are you? Is your girlfriend an engineer or designer?( I ask because she seems to be sure it will cost too much). How big of a dome you want? Are you going to use regular utilities from your county or city(extra paperwork and permits to connect to these services). Now,.. given that you get the land, have the money, the time and get the permits(if necessary), and given that you have some skills or can go slow to learn them , then yes, if it is a small project I say you could build it yourself. Most of the time you will need help. If you plan it well, there may be stages that you get hired help for specific days and tasks, and then there are other things that you can do yourself to save money, even if they take you longer. Once you get your foundation and dome built( help needed), then you can install the plumbing, electricity, walls, etc by yourself. Everything is doable. Some things just require more time, money and effort than others. If you have enough money and space to host a domegaia workshop, they might build it for you and you do your part of providing what they need. There are many options.